Mass spectrometry is a technique used in the field of chemical analysis to detect and identify analytes of interest. Such analytes include, but are not limited to, residues and vapors from explosives, chemical warfare agents, toxic chemicals, narcotics, volatile and semi-volatile organic compounds, airborne contaminants, food and beverage contaminants, and pollution products. In use, a sample is ionized so that components may be acted on by magnetic fields, electric fields, or combinations thereof, and subsequently detected by a detector.
As chemical analysis has become a more routine part of many industries, a need has developed for smaller, lighter mass spectrometers that can be incorporated more easily into laboratory, medical, security, and industrial settings and that have lower initial instrument costs and continued operating costs. Mass spectrometers employing ion traps are more easily miniaturized than other structures such as quadrupole, time-of-flight, and sector mass spectrometers. Because of their small size, they may be used in both stationary and portable (field deployable) mass spectrometry applications.
An ion trap is a device that uses an oscillating electric field to store ions. The ion trap works by using an RF quadrupolar electric field that traps ions in two or three dimensions. A 3-D ion trap such as, for example, a cylindrical ion trap, may include a ring electrode disposed between a pair of end cap electrodes. In a cylindrical ion trap, the ring electrodes and end cap electrodes may define a cylindrical interior region.
One technique for creating ions from neutral sample molecules is called electron ionization. In this technique, an electron beam is accelerated by an electric potential, may be focused by a lens, and introduced into the trap via an aperture in an entrance end cap electrode to ionize a sample contained within the trap. Ions are then sequentially ejected from the ion trap based on their mass via an aperture in the exit end cap electrode. Ions are selectively ejected in this way by adjusting the RF electric field inside the trap in a controlled manner. A mass spectrum can then be generated by measuring the ejected ions with a detector.
In conventional cylindrical ion traps, the ejection efficiency, resolution, and/or sensitivity of the mass spectrometer may suffer due to the configuration of the trapping elements. For example, in cylindrical ion traps having RF electric fields, the ions may be trapped between symmetric end cap electrodes. When the ions are excited to perform mass-dependent ejection from the trap, the size of their orbit may increase equally toward each end cap electrode. As a consequence, some ions may be ejected via the entrance aperture, reducing the overall sensitivity of the device.
Other ions may actually miss the exit aperture. Such ions may deposit on the exit end cap and, over time, form a resistive layer around the exit aperture due to the deposited material. The resistive layer may subsequently accumulate and hold a charge that distorts the field in the trap which, in turn, may reduce instrument performance in the form of reduced sensitivity, mass range, and resolution. While many mass spectrometers are laboratory instruments with sophisticated users who have both knowledge and access to tools and cleaning agents, disassembly and cleaning may be impractical or impossible in portable mass spectrometer devices that may be deployed outside the laboratory.
Additionally, because of the symmetric configuration of conventional cylindrical ion traps and the geometric dimensions of the end cap apertures, some electrons injected into the trap hit an area of the exit end cap electrode around the exit end cap aperture. This leads to potential contamination or degradation of the exit end cap electrode, which can cause similar effects on performance.
Finally, mass spectrometers employing cylindrical ion traps may have poor spectral resolution compared to spectrometers employing other types of ion traps unless special techniques are implemented in their design. The spectral resolution refers to the ability to differentiate spectral peaks of similar mass-to-charge ratio. The spectral resolution is typically measured as the ratio of a spectral peak's mass-to-charge value divided by the width of the peak at half its height or full-width-half-max (FWHM).
In other ion traps (e.g., hyperbolic traps) the spectral resolution may be improved by adding a hexapole field component to the ion trap. The hexapole field component may be created by, for example, changing a curvature of an inner surface of one or both of the end cap electrodes. This technique is not practical with cylindrical ion traps having flat electrodes, without adding cost and complexity to manufacturing the ion trap. Another technique for generating the hexapole field in hyperbolic traps is to vary the space between one of the end cap electrodes and the ring electrode. This technique may generate other undesirable fields within cylindrical ion traps.
The present disclosure is directed to a mass spectrometer that addresses one or more of these concerns.